1. The Field of the Process
This process relates to the synthesis and stabilization of metal-containing nanoparticle colloidal dispersions. More specifically, this process relates to an improved, low temperature, chemical solution phase synthesis of period 4 and 5 elements of the Periodic Table of the Elements and the stabilization of their associated colloidal metal and alloy dispersions.
2. Background Art
There is a considerable interest in the synthesis and chemical/physical properties of small metal particles and their alloys in the nano-domain where particle sizes are less than 100 nanometers. Application in the areas of photonics, medicine, coatings, additive manufacture and printable electronics are viewed as especially attractive commercial venues for these materials. Among the many synthetic routes to these materials, solution phase reaction is an especially straight-forward, economical and versatile method that also provides facile compositional materials engineering and particle size control. In one embodiment of this technique, metal particles are generated by the reduction of an organic metal salt in a solution phase (aqueous or organic) by a reducing agent with concurrent stabilization of the metal colloidal suspension with a chemical “capping agent” or stabilizer. Stabilizers can carry a formal electronic charge or not or can belong to a class of molecules called surfactants (composed of a lipophilic and hydrophilic moieties. Generally, capping agents, ligands, “ligate” or bond to the underlying metal with something less than a two electron bond.
A considerable amount of technical and patent literature has evolved around this technique with particular emphasis on the noble metals group (Platinum, Gold, Silver, Rhodium, and the like) as these materials, even in the nano-state, are resistant to aerial oxidation. Their electrochemical reduction potentials occur at large, positive voltages so the forward reaction M2++2e−→M0 is favored by a robust, spontaneous negative free energy change. For example the E0 of Au is 1.692 volts and that of Pt is 1.18 volts.
Unfortunately, this is not the case for most of the period 4 and 5 transition metals such as Ti, Fe, Co, Ni, Cu, Zn, as the opposite situation prevails. Their E0s are either small positive values or are negative and large in magnitude (E0 Ti=−1.63 V, E0 Ni=−0.25 V, E0 Cu=+0.34 V and E0 Zn=−0.76 V), which means the backward or oxidation reaction M0→2e−+M2+ is highly favored. Thus, it is difficult to find or synthesize these metals in a free and reduced state, as they are more commonly found as oxides due to the ease with which they combine with oxygen. The ubiquitous materials paint (i.e., TiO2), sunscreen (i.e., ZnO), and red (Cu2O) or black (CuO) are commonly encountered examples of these oxides.
In the preparation of fully reduced, referred to as “zero valent,” nano-metals, it is common practice to use very high concentrations of polymeric ligands (molar ratios of >15:1 of organic stabilizer: metal) present in the metal cation reduction step to both sterically and electrostatically stabilize the resulting colloidal dispersion (to prevent agglomeration settling) and to provide a protective sheath that excludes, to varying extent, oxygen from the metal particle surface. This high concentration regime becomes more extreme when high particle loadings are desired—usually in the range that is equal to or greater than 20%. Herein lies the crux of the problem. Having stabilized the colloidal suspension with large quantities of complex organic molecules, the subsequent intended use of the material (conductive trace, Radio Frequency Identification (RFID) tags, electronic components, catalyst, coating, and the like) requires one to use extreme conditions of heat or light, long times or complex and involved reaction sequences to remove the protective sheath and render the metallic material reactive and useful for its intended purpose. This type of stabilization will be referenced herein as “thermodynamic stabilization” and it usually provides stability against settling and oxidation for periods of months. The opposite approach to this method could be called “kinetic stabilization” which requires better mixing, occurs with rapid reaction times and lower temperatures. Kinetic stabilization works for periods of days, allows for, and perhaps requires, subsequent chemical functionalization of the particle surface.
A very complete and recent review of copper and copper based nanoparticle synthesis with application to catalysis is given by Gawande et al. in Chem. Rev. 2016, 116, 3722-3811. Table 1 contains 107 entries detailing wet chemical methods (solution reaction chemistry) and covers: solvents (water, acetone, ethanol, proanediol for example), stabilizers (oleic acid, olylamine, polyvinyl pyrrolidone, polyethylene glycol, aerosol OT—a surfactant), reducing agents (hydrazines, sodium borohydride, ascorbic acid, glucose), copper chemical precursors (halides, acetates, nitrates, sulfates, acetylacetates) and thermal treatment conditions (usually −100 degrees Celsius, for an hour or so). Additionally, the review of Tan and Cheong, J. Nanopart Res (2013)15:1537, discussed both copper and silver synthetic routes and focused exclusively on chemical reduction. They categorized the stabilizers as: polymers or surfactants (Triton X-100, Tween 20, PVP, PVA, Sodium Dodecyl Sulfate, etc.), acids (mercaptoacetic acid, sodium citrate, EDTA, stearic, cholic) amines (oleylamine, pheylenediamine, aminosilane), bromides (CTAB, DTAB, TTAB, TBAB), ligands (thiolates, sulfonates), alcohols (isopentanol, dodecanethiol) and silicates (Laponite).
Specific literature and patent examples of commonly used named stabilizers or capping agents include: PVP (Zhang et al. Nanoscale Res Lett (2009) 4:705-708 “Facile Fabrication of Ultrafine Copper Nanoparticles in Organic Solvent”), surfactants (discussed above), alcohols (Kawaska et.al. WO 2015129466 A1), diols and ether alcohols (Kurihara et al. U.S. Pat. No. 7,033,416 B2, Apr. 25, 2006), amines (U.S. Pat. Pub. No. 2015/0099172 A1, Apr. 9, 2015 “Synthesis of Metal Nanoparticles”), polyethylene amines (Quiintea et al. WO2015082530 A1, 2015), amino acids (Yu et al. Nanoscale, 2015, 7, 8811), carboxylic acids (Abe et al. JP 2011032558 A 20110207, 2011), and dithiacarbonates (Chopra, et al. U.S. Pat. No. 7,976,73362B2). Zhong et al. in U.S. Pat. Pub. No. 2008/0278181A1 use a combination of surfactants, oleyic acid and oleyl amine as capping agents in the high temperature (>145 degrees Celsius) and extended time (˜30 min) synthesis of claimed oxidation resistant copper nanoparticles 5 nm and greater. It should be noted that a separate isolation and re-suspension step was required to make the final dispersion. The inventors Alfred Zinn and P. Lu in U.S. Pat. No. 9,378,861 B2 uses an amine surfactant of carbon chain length C6-C18 and a ligand N,N′-dialkylethylenedaimine to stabilize Cu nano particles with a poly disperse 1-10 nm size-frequency distribution. No direct sizing data are provided, other than a 100 nm scale transmission electron microscope (TEM), although the preferred size range is claimed to be 3-5 nm. These Cu nanoparticles are deemed to be metastable and have been previously formed in the presence of the alkylated ethylene diamine reducing agent at low temperature (<60 degrees Celsius). However, the particles sinter or “fuse” into a conductive circuit element in the 100-200 degrees Celsius temperature range and under pressure (<95 PSI) which is claimed to not cause additional heating. Thus a combination of both pressure and temperature are required for fusing the nanoparticles. Interestingly, short organic chain copper precursors (glycolates and separately copper acetate with formic acid and cyclohexylamine) have been described in RSC Adv., 2014, 4, 60144 by Wen-dong Yang, et al. however, these inks require very high temperatures (˜220 degrees Celsius and 290 degrees Celsius) to achieve reasonable electrical conductivity.
The naturally occurring polymer chitosan (Usman et al. Int. J. Nanomedicine 2013:8 4467-4479) is an interesting and economical material but is difficult to use due to its limited solubility in most practical solvents. The use of negatively charged stabilizers such as carboxylic acids, poly acrylic acid, EDTA and the like is fraught with difficulties as the stabilizer will bind much more strongly to the starting metal cation (positively charged precursor) than it will to the final fully reduced metal, thereby thwarting the reduction process. This antagonistic and undesired effect can be overcome by the brute force application of extremely excessive reducing agent levels (greater than 10× the required stoichiometric amount) to bias the reaction towards the production of the desired product, zero valent metal.
Bi-dentate capping agents (not surfactants) feature two metal binding sites on the stabilizer molecule usually in close proximity (e.g., <4 atoms removed from each other). Prominent among these are the bis nitrogen materials 1,10 phenanthroline. In U.S. Pat. No. 7,335,245 B2 He et al. Honda Motor Feb. 27 2008 “Metal and Alloy Nanoparticles” teaches multicomponent metal alloys invariably incorporate a noble metal, are supported on a carbonacious material, and are heated to several hundred degrees Celsius in a high temperature boiling solvent. Other bidentate containing materials featuring two nitrogens are bipyridine (BiPY) derivatives (U.S. Pat. Pub. No. 2014/0212497 A1 Pikramenou et al. Jul. 31, 2014 “Coated Nanoparticles” teaches a composite nanoparticle in which a first noble metal nanoparticle is functionalized by at least one type of metal complex and surfactant. The preferred metal complexes are those of Ir or Ru and combined with various derivatized bipyridines (“BiPY”) and phenanthrolines). The limitations of these approaches are the obvious cost and scarcity of the precious metals involved and the financial and time expense of making “boutique” derivatized BiPY or phenanthroline compounds. It is not clear that the derivatized materials offer that much more benefit than the parent compounds. Off-the-shelf diamines, whose nitrogen atoms are located not immediately adjacent (as in 1,2) but in a 1,3 or 1,5 or even 1,7 disposition is described by H. Takamatsu in U.S. Pat. No. 6,197,366 B1 (2001) in for the formation of coatable, viscous, and homogeneous metal containing pastes. Very high temperature (>300 degrees Celsius) processing and long times (˜5-10 minutes) were required to achieve pure metal film strips.
Nitrogen atom-containing molecules are not the only bi-dentate materials. The bi-dentate binding sites of a capping agent could feature two different atoms as is the case for the aminopropanol. Hokita et al. in ACS Appl. Mater. Interfaces 2015, 7, 19382-19389 describes the use of 1-amino 2-propanol and 1-amino 3-propanol as capping agents in the production of copper based inks. Very high levels of capping agent relative to the amount of metal copper were employed (a 10:1 molar ratio was cited) as were high levels of the reducing agent hydrazine (10:1 which is a 19 fold excess on an electron equivalent stoichiometric basis) which yielded upon sintering a fairly electrically resistive trace, 30 microohm-cm after a prolonged heating time of 15 minutes at high, 150 degrees Celsius, temperature. For comparison, the bulk resistance of unannealed copper is 1.68 microohm-cm. Other variants on the aminoalcohol capping agent theme are; Suguiyama et al. J. of Mat. Sci: Materials in Electronics 2016 who show that 2-amino-1-butanol has a capping agent has superior copper adhesion properties to polyimide films relative to the above cited amino propanols, Farraj et al., Chemical Communications (Cambridge, United Kingdom) 2015, 51(9) 1587-1590 who claim that 2-amino-2-methyl-1-propanol capped copper inks only require 140 degrees Celsius under a N2 blanket to cure on a flexible substrate and finally that a N2 blanket is not required to prevent oxidation during the curing of a 3-dimethylamino-2-propanol or dimethyl ethanolamine capped copper ink as reported in by Kang et al. in Repub. Korean Kongkae Taeho Kongbo 2014 KR2014111070 A20140918.
Various strategies to mitigate the extent of oxidation for copper nanoparticles (in particular) are discussed in Materials 2010, 3, 4626-4638 by Magdassi et al. Essentially they involve putting a protective coating on the particles thereby making a core:shell structure. The copper coatings on these particles are: organic polymers (whose limitations have already been discussed), carbon or graphene, or an inert material such as silica or metal (typically binary combinations of Au, Pt, Ag, Pd). The precious metal shell approach to oxidation resistance suffers from obvious cost limitations. Specific core-shell patents are: Shim et.al. U.S. Pat. No. 7,611,644 B2 Nov. 3, 2009 which claims a precious metal shell technology; Chretien et.al. U.S. Pat. No. 7,749,300 B2 Jul. 6, 2010 claims a photochemical method of making a bi-metallic core-shell nanoparticle; Lauterbach et al. U.S. Pat. Pub. No. 2013/0288892 A1 Oct. 31, 2013 teaches core-shell nanoparticle synthesis under a reducing gas environment; Kim et al. U.S. Pat. No. 7,625,637 B2 December 2009 teaches metal nanoparticles formed from low reduction potential metals (core shell structures with silver and palladium at the core and nickel shells) and finally Harutyunyan et al. U.S. Pat. No. 8,088,485 B2 claims that alumina or silica supported metal alloyed nanoparticles can be formed by heating a mixture of metal acetates in passivating solvents such as glycol ethers or 2-(2-butoxyethoxy) ethanol.
It should be noted that care needs to be taken in accepting the conclusions that fully reduced metal particles were synthesized in works where the identity of the nanoparticle has not been established, by either electron diffraction, x-ray diffraction or visible spectroscopy (usually by the observation of a 580 nm absorption peak in the case of copper, see, G. Mie Ann. Phys (Liepzig) 1908, 25,377). False product identification may be a problem when sodium borohydride is used as the reducing agent, as it is well known that the borides CrB2, Co2B and Ni2B and by extension CuB, are readily produced by this reducing agent. Subsequent sintering will produce a broate or boric oxide coating which is a very good electrical insulator.
Metal ink formulations using aliphatic alcohols that are sinterable by heat or light are described for wet coating applications, Li et al. in U.S. Pat. No. 8,404,160 B2 Mar. 26 3013 or for jetting applications which use a combination of alcohols, diols and glycols and require specific surface tension and viscosity values, see M. Carmody in U.S. Pat. Pub. No. 2014/0009545 A1 Jan. 9, 2014. Again the limitation herein is the need for a high power laser to sinter the copper ink that has been previously applied to a surface, into a highly electrical conducting pattern or trace.
Despite the advances in the art, there is clearly a need for stabilization strategies and chemistries that facilitate the use of the as-made metal nanomaterials that does not require large quantities of complex organic polymeric molecules or excessive amounts of reducing agents while still providing high suspension density materials with good low temperature sintering properties that are reasonably electrically conductive.